Counters and sensors are used to detect light scattered by particles entrained in a stream of fluid, e.g., in an air stream. In general, such counters and sensors have a very-small-diameter, highly intense laser beam. The air stream and any particle entrained therein is drawn through the light beam. In the vernacular of the industry, the small spatial region formed by the intersection of the air stream and the laser beam is called the "sensor volume" or the "view volume."
When light strikes the particle, it is scattered. Reflectors direct the scattered light to a photodetector which emits an electrical current pulse, the amplitude of which is generally proportional to the intensity of the light impinging upon it. And, of course, light intensity is a measure of particle size.
Some types of sensors flow air along an enclosed transparent tube; others "project" the air and accompanying particles at a particular flow rate (often measured in cubic feet per minute) from one tube across an open space to another tube. In sensors of the latter type, there is no tube wall (however transparent such wall may be) to impair light scattering and collecting. In other words, the particle is briefly illuminated by a light beam as it "flies" through an open space.
Among other uses, particle counters incorporating particle sensors are used to obtain a measure of air quality by providing information as to the number and size of particles present in some specified volume of air, e.g., a cubic meter of air. Even work environments which appear to human observation to be clean--business offices, manufacturing facilities and the like--are likely to have substantial numbers of airborne particles. While such particles are not usually troublesome to the human occupants, they can create substantial problems in certain types of manufacturing operations.
For example, semiconductors and integrated chips are made in what are known as "clean rooms," the air in which is very well filtered. In fact, clean rooms are usually very slightly pressurized using extremely clean air so that particle-bearing air from the surrounding environs does not seep in. And the trend in the semiconductor and integrated chip manufacturing industry is toward progressively smaller products.
A small foreign particle which migrates into such a product during manufacture can cause premature failure or outright product rejection even before it is shipped to a customer. This continuing "miniaturization" requires corresponding improvements in clean-room environments (and in the related measuring instruments) to help assure that the number and size of airborne particles are reduced below previously-acceptable levels.
Factories making semiconductors and integrated chips are not the only sites at which particle sensors may be used. Makers of pharmaceutical products have applications for such sensors to help exclude foreign matter from medicines and drugs.
Particles which are of concern in clean room and pharmaceutical working environments range in size from a few microns down to so-called "submicron," i.e., 0.1 micron or even smaller. (To give an idea of relative size, it is said that a particle 10 microns in size is about as small as can be seen with the unaided human eye.)
And when operating a particle sensor in such environments, it is preferred to measure the number of particles of each of several different size ranges which pass through the sensor per unit time. The computerized part of the sensor is configured with "bins," i.e., counting registers which add a count based upon data for each of several sizes of particles. For example, the computerized bins may be configured to respectively count particles 0.1 to 0.2 microns in size, particles from 0.2 to 0.3 microns, particles from 0.3 to 0.4 microns and so forth.
Known particle sensors use both analog and digital techniques to determine certain aspects of particles passing through them. For example, U.S. Pat. No. 5,047,963 (Kosaka) discloses an apparatus configured for blood cell analysis. Data regarding the cell nucleus (rather than the entire cell which, compared to particles of concern in clean room analysis, is quite large) is processed using a digital signal processor.
U.S. Pat. No. 4,984,889 (Sommer) discloses a particle size measuring system which uses analog pulses from a photodetector. Such pulses result when a particle passes through the system. It is understood that only the peak amplitude of an analog input pulse is digitized and registered in a computer or the like to increment "counts" of particles of differing sizes. U.S. Pat. No. 4,623,252 (Hollenbeck) discloses a particulate counter which digitizes an analog output. A comparator compares the signal to a threshold voltage value and if the threshold is exceeded (which is interpreted to represent detection of a particle), a sampler performs over 200 samples of the A/D converter output. The single maximum sample is selected and the lesser samples are disregarded.
U.S. Pat. No. 5,424,558 (Borden et al.) also mentions using digital signal processing to perform particle analysis but there is scant information as to how this is done. The Borden et al. apparatus is capable of a type of dynamic tuning, i.e., adjustments to bandwidth and gain to accommodate different types of signal pulses.
While these and other known particle sensors have been generally satisfactory for their intended purposes, they are not without disadvantages. For example, it is highly preferred to be able to distinguish a particle which is to be sized and counted from some sort of "floater," i.e., a vagrant speck of material which passes through the light beam. Such a speck may have simply stuffed off of a tube or housing wall. Some counters, like that of the Hollenbeck patent, count signal peaks or crossings of a photodiode output voltage which exceeds some threshold voltage. In a counter of this type, a floater may produce several false counts.
Another disadvantage of known approaches is evident in the Borden et al. apparatus. Such apparatus cannot be dynamically reconfigured (reconfigured while in operation) to change the value and number of thresholds which define the particle size bins. For greatest flexibility in operation, this would be a desirable feature.
A new particle sensor which addresses disadvantages of known sensors would be an important advance in this field of technology.